This document concerns an invention relating generally to lightguides such as fiberoptic cable and edge-lit films, and more specifically to devices and methods for providing edge-lighting for films with high efficiency.
There are numerous forms of lightguides, with perhaps the most common being the optical fiber, which is typically formed as a clear glass or plastic cylinder with a diameter of 1 micrometer to 5 mm. A light source is coupled to an end of the optical fiber, and is transmitted through its length, typically with the intent of delivering as much of the light as possible to the opposite end of the optical fiber. Some of the light is lost along the length of the optical fiber owing to light absorption and light scattering (i.e., light escaping through the surface of the fiber). However, with the choice of appropriate materials and manufacturing processes, absorption and scattering losses can be minimized. For example, whereas an optical fiber formed of plastic (e.g., polymethyl methacrylate or PMMA) typically has an attenuation of less than 0.2 dB per meter, an optical fiber formed of high-grade fused silica typically has an attenuation of less than 0.01 dB per meter. Plastic lightguides tend to have greater losses, and thus tend to be used only in circumstances where light only needs to be transmitted short distances. Scattering can also be reduced by forming an optical fiber with a core having a higher refractive index, and an outer layer having a lower refractive index, so that light received by the core experiences internal reflection (i.e., it reflects from the boundary between the core and the outer layer and continues to travel along the fiber, rather than being transmitted from the core to the outer layer and its surroundings). Efficient light transfer is also enhanced if the light source is coupled to the end of the optical fiber with high efficiency so that the greatest possible amount of light from the light source is transmitted into the fiber. Good coupling efficiency can be achieved by (for example) treating the end of the optical fiber to be as smooth and transparent as possible, thereby better allowing the light to be transferred into the end of the fiber rather than being reflected therefrom, and by transmitting the light from the light source to the fiber using an optical coupling gel matched to the refractive index of the fiber.
Another form of lightguide is a transparent plate which has a light source coupled to one edge. This arrangement is often referred to as “edge-lighting,” and it typically requires plates with thicknesses greater than 2 mm to achieve effective coupling from typical light sources such as light emitting diodes (LEDs) and halogen, incandescent, metal halide or xenon lamps. The surface of a plate may be treated at certain areas, as by surface roughening, etching, or the addition of a material that promotes light scattering (e.g., white paint), to reduce or defeat internal reflection at these areas to cause light emission from the plate at these or adjacent areas. As a result, the treated areas appear to glow. Because the plates typically do not efficiently receive or transmit light, and have higher losses along their lengths, the treated areas must often be functionally graded (i.e., they must generate lesser scattering near the light source and greater scattering farther from the light source) if the treated areas are to appear to have uniform illumination.
The plates are typically rigid, having very limited flexibility, though in some cases flexible plastic films are used (typically having a thickness of 0.5 mm or so). However, these are rarely used because it is difficult to efficiently couple light into such films at low cost. Most light sources have dimensions greater than millimeters, with the films having much smaller thicknesses for receiving the input light, so it is difficult to efficiently and inexpensively channel the majority of the light source's light output into the edge of the film. One solution to this problem is presented in U.S. Pat. No. 7,237,396, wherein a light source is coupled to the first ends of a bunched bundle of optical fibers, and the second ends are spread along an edge of the film to effectively provide an array of input light sources. The drawback of this approach is that it can be time-consuming and difficult to achieve: for efficient coupling, the second ends of the fibers must be precisely aligned with the edge of the film; treatment of the fiber ends and film edge to reduce scatting is time-consuming; and similarly the assembly demands of the system (which preferably uses optical coupling gel at the various optical interfaces) are high. There are also losses at the interface between the light source and the bundle, since the spaces between the fibers in the bundle create a loss. It would therefore be advantageous to have devices and methods available for high-efficiency coupling of light sources to films and plates with lower cost and ease of manufacture and assembly.